NERVOUS SYSTEM. 



hemispheres dorsad of the 'tween-brain. Ventrad of the corpus 

 callosum another secondary union has resulted in the produc- 

 tion of the fornix (Fig. 143, 11). 



5" 6 ,/ 



Z- 



a 



FIG. 144. LATERAL SURFACE OF THE BRAIN. 



a, olfactory bulb; 3, cgrebral hemisphere; c. cerebellum; </, medulla, oblpngata ; 

 e, tractus olfactorius; _/". lobus pyriformis or tractus postrhmalis; g, pons; //. trape- 

 zium; i, pyramid; /, area elliptica; k, area ovalis. //, N. opticus; V, N. trigemi- 

 nus; VII, N. facialis; VIII, N. acusticus; IX, N. glossopharyngeus; X, N. vagus; 

 XJ, N. accessorius. I. sulcus praesylvius; 2, sulcus cruciatus; 3, sulcus ansatus; 

 4, sulcus lateralis; 5, sulcus suprasylvius; 6, sulcus anterior; 7, sulcus posterior; 8, 

 sulcus. rhinalis; 9. fissura Sylvii; 10. sulcus rhinalis posterior. 



External Features. With increase in size the mass of the 

 cerebrum shows externally a tendency to divide into three 

 lobes, one craniad, the frontal (Fig. 145, A); one caudoven- 

 trad, the temporal (B) ; and one caudodorsad, the occipi- 

 tal (C). The two latter are not distinctly marked off from one 

 another. The limit between the temporal and frontal lobes is 

 marked by a short deep fissure, the lateral fissure (fissura 

 cerebri lateralis), or fissure of Sylvius (Fig. 144, 9; Fig. 

 145, a). Each lobe is thrown up into elevations or gyri, 

 which are separated by grooves or sulci ; these are described 

 below. The homology of the cerebral gyri and sulci of the 

 cat with those of man is in most cases uncertain. 



The sulci and gyri of the cerebral hemispheres may be 

 described briefly in their main features as follows: On the 

 lateral surface of the hemisphere (Fig. 145) the lateral fissure 

 (fissura cerebri lateralis), or fissure of Sylvius, separating 

 temporal and frontal lobes, forms the most convenient point 

 of departure for an understanding of the fissures. The fissure 



